A process for the coating objects is known from WO 03/087198, in which a solution or dispersion of the additive and the binder are contacted with the polymer particles by spraying the dispersion, for instance via a nozzle or with the aid of a propellant. After that, a liquid is sprayed that has a cleaning effect on the spraying system in the presence of the plastic pellets.
The drawback of this process is that a portion of the composition ends upon the wall and is deposited on any parts of equipment present in the space in which the composition is sprayed. This makes it necessary to repeatedly clean the space in which spraying takes place. This holds in particular when batches with different additives are consecutively processed. In that case, cleaning must take place after each change of additive.
The aim of the invention is to provide a process for the coating of objects that to a significant extent prevents fouling of the space in which coating takes place.
This aim is achieved according to the invention in that the objects are kept in mutual motion while they are being contacted with the composition and while the additive-containing layer of binder is being formed and consolidated and that the process is carried out in a container bounded by walls of which the temperature is so much lower than the application temperature that the formation of a film of the binder on the walls is prevented.
It has been found that the walls, by keeping them at a temperature that is lower than a temperature at which formation of a film of the binder on the wall is prevented, are not fouled but that any deposits of the composition on the walls do not adhere to the walls and are readily taken up again by the moving particles when they come into in contact with the walls. This effect is achieved at any rate when the temperature of the walls is below the minimum temperature at which the binder can form a film. However, in many cases this requirement is unnecessarily restrictive and often it will suffice to keep the walls at a temperature that is lower than the dew point of the optional distributing agent during application and consolidation. Under those conditions a certain degree of condensation of the distributing agent on the colder wall surface will take place so that drying in of the composition on the wall is prevented. As a result, a large number of batches of objects can be coated with the same additive without intermediate cleaning, and it has even found to be possible to do without cleaning also when there is a change of additive.
With the process according to the invention it is possible to coat objects of random shape and of any material that is resistant to a temperature that is higher than the minimum temperature at which the binder to be used can form a film. The process is most suitable for the coating of objects without cavities or protruding parts, such as for instance seeds, pills and pellets. Such objects can be processed below the temperature at which degradation, decomposition and other undesirable changes in their properties start to occur. As softening temperature Tp of the polymer use is made of the glass transition temperature for amorphous polymers and of the melting point, determined by means of DSC with a heating rate of 10° C. per minute, for semi-crystalline and crystalline polymers.